As compared with an imaging device using a CCD (Charge Coupled Device), an imaging device using a CMOS (Complementary Metal Oxide Semiconductor) enables faster readout of pixel signals, less power consumption, and higher integration. Since such an imaging device can comply with requirements placed on an imaging apparatus in terms of size, performance, and the like, it has been attracting attention as an imaging device to be mounted on an imaging apparatus. Such a CMOS imaging device is also unique in enabling a so-called random scan whereby electric charge is read out by specifying an arbitrary pixel via horizontal and vertical signal lines.
With regard to an imaging apparatus incorporating such a CMOS imaging device, Japanese Unexamined Patent Application Publication No. 2000-152057 discloses a configuration in which the front curtain in a shutter operation is effected by an electronic focal planer shutter, and the rear curtain is effected by a mechanical focal plane shutter. That is, according to the disclosed imaging apparatus, reset signals for resetting respective pixels included in the imaging device are sequentially supplied on a pixel line by pixel line basis to start exposure on the imaging device (electronic focal plane shutter as the front curtain), and mechanical light shielding is effected by causing a curtain to travel after the elapse of a set exposure period (mechanical focal plane shutter as the rear curtain), thus terminating exposure of the imaging device.
Incidentally, more and more recent imaging apparatuses such as digital still cameras incorporate a camera shake correction function to suppress a deterioration in the image quality of a captured image due to camera shake. With this camera shake function, an amount of shake applied to an imaging apparatus is detected by an angular velocity sensor or the like, and a structure such as an imaging device undergoes a shake correction drive in accordance with the amount of shake. However, a problem that often occurs with this kind of imaging apparatus incorporating a camera shake correction function is that the camera shake correction performance deteriorates due to vibrations generated by the traveling curtains of the mechanical focal plane shutter. This problem occurs as follows. When the curtains travel so as to open the optical path and the opening of the optical path is completed, the curtains collide against each other within the shutter unit, generating impact vibration (shutter stopping vibration). Because this impact vibration affects a camera shake correction drive mechanism situated in close proximity, that is, because a force different from that for camera shake correction is applied to the camera shake correction drive mechanism due to the impact vibration, a displacement from a predetermined correction position occurs, causing misoperation. In particular, since no fixation by locking or static friction is performed during the shake correction drive, the influence of this problem becomes significant.
Japanese Unexamined Patent Application Publication No. 7-20523 discloses mitigating the above-mentioned impact vibration by controlling the curtain speed of the mechanical focal plane shutter to be slow during long exposure for which camera shake correction is required. However, since controlling the speed of curtains requires a rather complex mechanism, this involves the problem of hindering downsizing of the imaging apparatus and driving up cost.
Although the above-mentioned problem of impact vibration can be overcome by applying the technique disclosed in Japanese Unexamined Patent Application Publication No. 2000-152057 so that the front curtain is always effected by an electronic focal plane shutter, this brings about another problem of exposure unevenness resulting from the fact that the front curtain is effected by an electronic focal plane shutter and the rear curtain is effected by a mechanical focal plane shutter. That is, the travel characteristics of the curtains of a mechanical focal plane shutter vary with a change in temperature/humidity, a change of posture, secular changes, and the like. Although no serious problem is likely to occur if both the front curtain and the rear curtain are mechanical focal plane shutters because both the front curtain and the rear curtain undergo substantially the same variations, since an electronic focal plane shutter does not undergo such variations in travel characteristics, exposure unevenness occurs due to a difference in curtain speed between the front curtain and the rear curtain. Such exposure unevenness becomes particularly noticeable when shooting at fast SS (shutter speed) during which slit exposure occurs, which makes it impossible to achieve stable exposure.
The present invention has been made in view of the above-mentioned circumstances, and accordingly it is an object of the present invention to provide an imaging apparatus which incorporates a CMOS imaging device and performs shake correction by driving the imaging device, in which the drive of the shake correction function section, that is, the shake correction drive is practically unaffected by the influence of vibration due to curtain travel of a mechanical focal plane shutter so that shake correction can be performed in an appropriate manner.